What necessitates a market is the act of buying and selling for the two works hand-in-hand and in a scenario where there's no seller then the process of buying is frustrated and that's no good market at all as it either mean that there's a problem somewhere. That's why the statement " if there's no seller there's no buyer is a fact and not a hypothesis.

Using this analogy: Just like a tree, the seller is like the root of the tree and the buyer the leaves. Where there's no tree roots there would be no leaves of a tree.